1. Field of the Invention
The present invention relates to an LED driver circuit, and in particular to a circuit for protecting an LED or similar lighting device from transient voltages in, for example, a vehicle electrical system.
2. Background Information
The instrument panel, center console and/or dashboard of a vehicle, such as an automobile or a truck, typically contains a number of switches for activating various components of the vehicle, such as, without limitation, the headlights, hazard lights and front and rear fog lights. A light emitting diode (LED) or other suitable light source is often provided in connection with each such switch in order to illuminate the area surrounding the switch assembly.
In addition, during operation of a vehicle, relatively large transient voltages may occur. These transient voltages are of relatively short duration and may result from a loose battery connection or other causes. If electrical devices on the vehicle, such as the LEDs described above, are exposed to the relatively large transient voltages, the devices could be damaged (all devices and components are on the same electrical system). LEDs used in conjunction with switch assemblies can typically have a maximum current rating on the order of 25 mADC, meaning they can only safely operate, without being damaged, at currents at or below such values. Thus, LED protection circuits have been developed for protecting LEDs from potentially damaging transient voltage spikes.
As is known, vehicles typically employ either a 12 volt or a 24 volt (larger vehicles) electrical system, and the prior art LED protection circuits that have been developed have been designed to operate only in the particular electrical system (12 volt or 24 volt) in question. FIG. 1 is a circuit diagram of LED protection circuit 5, which is one example of a prior art LED protection circuit for use in connection with a 24 volt vehicle electrical system. As seen in FIG. 1, the 24 volt electrical signal (actually, the signal may typically range from between 18 volts and 32 volts) is applied, when appropriate, across terminals 10A and 10B. LED protection circuit 5 includes resistors 15 and 20 connected in parallel, each of which is preferably a 3K Ω resistor. Zener diode 25 is provided between resistor 15 and terminal 10B, and is chosen such that it will not begin conducting current until an upper threshold voltage is reached, such as about 36 volts. Zener diode 25 may be a 39 VDC zener diode, which will typically begin to conduct at about 35–36 volts. LED 30, which is the LED to be protected by the LED protection circuit 5, is provided between resistor 20 and terminal 10B. Also, resistor 35, such as a 51 Ω resistor, is provided in series with LED 30. In operation, if an appropriate 24 volt (actually 18 volt to 32 volt) signal is applied across terminals 10A and 10B (such as when the associated switch is in an “on” position), zener diode 25 will not conduct current, and the voltage will be applied across parallel resistors 15 and 20 and LED 30. As a result, a current of about 16 mADC will flow through LED 30, which is within the operating range of LED 30. If, however, a voltage of about 36 volts or greater, as a result of a transient condition, is applied across terminals 10A and 10B, then zener diode 25 will conduct and will clamp the excess voltage down and shunt harmful current away from LED 30, thereby protecting LED 30. Resistor 35 and diode 40 are provided to protect LED 30 in the event circuit 5 is improperly connected to the voltage source (i.e., backwards).
FIG. 2 is a circuit diagram of LED protection circuit 5′, which is one example of a prior art LED protection circuit for use in connection with a 12 volt vehicle electrical system. LED protection circuit 5′ is identical to LED protection circuit 5, except that the values of resistor 15′, resistor 20′, and zener diode 25′ are chosen to provide an appropriate current to LED 30 when a 12 volt (actually typically ranging from between 9 volts and 16–17 volts) signal is applied across terminals 10A and 10B, and to protect LED 30 from voltages that exceed about 18 volts. As will be appreciated, two different circuits are required, depending on the type of electrical system (12 or 24 volt) being employed. Suppliers and maintenance personnel must thus stock both types of protection circuits.
In addition, LED protection circuits such as those shown in FIGS. 1 and 2 are currently mounted to the switch assembly as shown in FIG. 3. Specifically, the LED protection circuit is implemented by attaching discrete electrical components 45 (e.g., the components shown in FIGS. 1 and 2) to a circuit board substrate material 50, such as FR-4, G-10 or the like. LED 30 is then attached to the circuit board substrate material 50 at appropriate electrical contacts. The circuit board substrate material 50 containing the LED protection circuit is then attached to a connector base 60 which forms a part of the switch assembly of the vehicle. Connector base 60 may be made of, for example, glass filled valox, and includes electrical connectors 65 for connecting the switch assembly to the vehicle electrical assembly. The problem with such a configuration is that the circuit board substrate material 50 is responsible for about a third of the price of the finished switch assembly as a whole (e.g., the circuit board substrate material 50 typically costs on the order of $0.50 with the cost of the whole switch assembly being approximately $1.50).